Such a scanning device is, for example, known from the DE 41 15 747 C2. The known apparatus comprises a radiation source, a rotatable plane-parallel prism, and a lens arrangement. The radiation source emits a light beam, which penetrates through the prism and which is subsequently projected via the lens arrangement onto a scene that is to be scanned. Thereby, the light beam is refracted on two refraction planes of the prism. Due to the parallelism of these two refraction planes, the light beam entering into the prism and the light beam exiting from the prism are offset parallel to one another by a value dependent on the angular position of the prism. Through rotation of the prism, this offset is varied, and the light beam is thus moved over the scene that is to be scanned.
The essential disadvantage of this apparatus exists in that the length of the path, which the light beam traverses from the radiation source to the lens arrangement, is dependent on the angular position of the prism, so that the light beam is defocussed by the rotation of the prism during the scanning process.